The present invention relates to power-supply arrangements for electronic circuitry and in particular to arrangements for minimizing ground currents.
A typical ground-current problem encountered in electronic circuits can be appreciated by reference to FIG. 1. FIG. 1 depicts, in conceptual form, a circuit module 10 at a first location that produces a signal having high-frequency components and transmits it over a signal medium 12 in the form of a coaxial cable to a remote location, where the signal drives a load 14. The coaxial cable 12 has a center conductor 16 and an outer, shield conductor 18, both of which are connected to the circuit module 10. The shield conductor 18 is connected both to a signal-reference, or output-ground, node 20 on the circuit module 10 and to a ground node 22 at the remote location. The coaxial cable acts to localize the fields associated with the transmitted signals so that radiation loss and interference are minimized.
FIG. 1 further depicts circuitry for supplying power to the circuit module 10. A transformer 24 steps voltage down from a 110- or 220-volt AC power source. In the illustrated circuit, the resultant stepped-down voltages are applied through two rectifier bridges 25 and 26 to respective voltage regulators 27 and 28. The voltage regulators produce positive and negative regulated voltages referenced to a power-supply ground 30. All of this power-supply circuitry may supply power to many circuit boards in a circuit cabinet.
In the absence of appropriate precautions, this type of arrangement can result in a ground loop. For example, suppose that load 14 is located on a circuit board mounted at a different location in the cabinet that contains module 10 and that that circuit board is powered by the power supply depicted at the left of FIG. 1. In such a situation, ground nodes 22 and 30 ma be connected to each other through the cabinet chassis or the ground planes of other boards, so there may be a very-low-impedance path, external to the module 10, between the two nodes. Since the signal transmission along the coaxial signal medium 12 necessitates some current flow in the shield connector 18 and thus some potential difference between its ends, there is a potential difference between ground node 20 in the circuit module 10 and ground node 22 at the remote location. Now, if nodes 20 and 30 were simply different points on the ground plane that contains module 10, this potential difference would impose a potential difference between nodes 22 and 30 that could result in high current flow in the external low-impedance path from ground node 22 to ground node 30. The resultant fields and ground voltages would be undesirable since they could be significant noise sources. This would be the result of a ground loop; there would be a circular low-impedance path from node 20 to node 22 and node 30 and back to node 20 that allows module 10 to drive not only cable 12 and load 14, as it should, but also the external path, which it should not.
To avoid this problem, designers have employed "split grounds" to break the ground loop. For instance, in the typical case in which the loop results from an external path to the ground node of a common power supply, a module containing output circuitry that might otherwise drive current through the external path is so arranged as to provide no direct low-impedance path between the power-supply ground node (such as node 30) and the module's output ground node (such as node 20), and it permits the output ground node to "float" with respect to the power-supply ground node, i.e., to assume the voltage level that causes no significant current flow in the external path. In such an arrangement, communication across the boundary where the loop has been "broken" must occur in ways that do not rely on ground references. For instance, communication could occur by optical coupling or, as FIG. 1 illustrates, by differential signals.
Specifically, the module 10 of FIG. 1 breaks the loop so as to isolate two devices 32 and 34 electrically from three other devices 36, 38, and 40. Transmission occurs across the boundary 42 between the resultant circuit segments by way of differential signals; that is, device 34 transmits a signal by way of two conductors 44 and 46, which are connected to a difference-mode device 36 that responds to the voltage differences between the signals rather than to the voltage difference between a single conductor and a common ground node.
Although this approach is effective, it tends to be expensive and space-consuming. The reason is that it requires separate power supplies for most circuit boards. FIG. 1 represents such separate power supplies as the output nodes 48 and 49 of separate transformer secondaries and bridge circuits (not shown) connected to a pair of opposite-polarity voltage regulators 50 and 51 corresponding to similar circuits 27 and 28. Since such a separate power supply might be required for every circuit board in a circuit cabinet, the expense and space penalties can clearly be significant and, in some cases, prohibitive.
One way to avoid such expense and space penalties is to employ DC-to-DC converters, whose DC output circuitry is electrically isolated from their DC input circuitry. The converters may all be powered by a common supply and in turn provide power to the output circuitry. DC-to-DC converters thus yield the intended result without employing separate power supplies.
However, the DC-to-DC-converter approach is limited in its range of applications. The reason for this is that a DC-to-DC converter employs an oscillator powered by the input DC voltage, and the oscillator output is magnetically coupled to a rectifier/regulator circuit to produce the electrically isolated DC output voltage. For the magnetic coupling to be performed efficiently in a small space, the oscillator must operate at a high frequency, so it is a significant noise source in the circuit. Accordingly, the DC-to-DC-converter approach is applicable only if such noise can be tolerated or large-size converters are acceptable.